TRAIL is a member of the TNF family of proteins, which also includes TNF-α and Fas ligand (1). These proteins are potent inducers of apoptosis. To date, five receptors for TRAIL have been identified, two of which, DR4 (TRAIL-R1) and DR5 (TRAIL-R2) (2–7), are capable of transducing the apoptosis signal while the other three DcR1 (TRAIL-R3), DcR2 (TRAIL-R4), and osteoprotegerin (OPG) do not transduce the apoptosis signal (8–12). All five receptors for TRAIL share significant homology in their extracellular ligand binding domains. Similar to Fas and TNF receptor I (hereinafter referred to as “TNFRI”), the intracellular segments of both DR4 and DR5 contain a death domain, and transduce an apoptosis signal through a pathway that involves the Fas-associated death domain protein (hereinafter referred to as “FADD”) and caspase 8 (6,7). In addition to transducing the apoptosis signal, the DR4 and DR5 receptors can also activate a pathway involving NFκb (6,7).
The biological functions of TRAIL that have been demonstrated include the capability of TRAIL to selectively induce apoptosis of transformed tumor cells, with normal cells being relatively resistant to TRAIL-mediated apoptosis (13–15). This selectivity suggests that, in contrast to Fas ligand, the administration of TRAIL is associated with very low levels of toxicity as demonstrated by systemic administration of TRAIL in an animal model without inducing significant toxicity (13). Thus, TRAIL has been proposed as a potent apoptosis inducing agent that would be a suitable therapeutic agent for the treatment of cancer and other diseases associated with abnormal cell proliferation. TRAIL also has been proposed to be a potent apoptosis inducing agent that would be suitable for the treatment of autoimmune and inflammatory diseases. It has been demonstrated that TRAIL-mediated apoptosis is involved in activation-induced cell death of T cells, thereby serving as an alternative mechanism to Fas ligand (16,17). TRAIL-mediated apoptosis may also function in the induction of apoptosis of T cells and other inflammatory cells (18), and plays a role in the killing activity of NK cells (19–21), and in the immunomodulatory function of dendritic cells (22,23). Thus, TRAIL-mediated apoptosis may also function in immunoprivilege and immunosurveillance.
The TRAIL receptor system is complex, and includes at least two death receptors, DR4 and DR5, and at least two non-apoptotic receptors, DcR1 and DcR2. All of these receptors not only share a high amino acid sequence homology, but also exhibit a similar binding affinity to TRAIL (2–12). The ability of the DcR1 and DcR2 receptors to compete for binding of TRAIL without inducing apoptosis suggests that they may act as decoy receptors that block or modulate the activity of the TRAIL ligand. Moreover, it has been reported that untransformed cells express higher levels of decoy receptors than do transformed cells. Thus, it has been proposed that the differential modulation of the expression of the death and decoy receptors may represent a key regulatory mechanism that determines the susceptibility of cells to TRAIL-mediated apoptosis, but due to the lack of receptor-specific antibodies (2). Although the expression and function of DR4 and DR5 have been studied extensively, progress has been impeded by the lack of receptor-specific monoclonal antibodies. The cell surface expression of DR5 has not been documented. It has been reported that a panel of anti-TRAIL receptor antibodies have been generated that are capable of inducing apoptosis of melanoma cells in vitro but only upon immobilization of the antibodies, to promote cross-liking, and, in some cases, the cells require culturing with actinomycin D (24). Several anti-DR5 antibodies have been generated (24). However, these previously generated anti-DR5 monoclonal antibodies have low apoptosis-inducing activity in vitro, even under the conditions of crosslinking. No in vivo activity has been reported. These antibodies have not been used for examining cell surface expression of TRAIL receptors (24). Thus, there exists a need for a monoclonal antibody selective for each specific TRAIL receptor that is not only able to bind to cell surface receptor but also to strongly induce apoptosis of various types of abnormal cells, including tumor cells, both in vivo and vitro without the requirement for crosslinking or immobilization. Such an antibody would not only provide potential therapeutic agent but also a diagnostic tool for functional analysis of TRAIL receptor. There exists a particular need for an antibody specific against each of the death inducing receptors DR4 and DR5.
In the development, or progression, of many diseases it is often the case that cells are not deleted. In many autoimmune diseases and inflammatory conditions, the surviving activated cells attack normal tissues or cells. Further, progression of tumorigenesis and the proliferative panus formation of rheumatoid arthritis are characterized by the unchecked proliferation of cells. Thus, insufficient apoptosis leads to the development of disease, and the uses of apoptosis-inducing ligand or agonistic monoclonal antibody to enhance apoptosis are considered as a potential therapeutic strategy for eliminating those unwanted cells.
For example, rheumatoid arthritis (hereinafter referred to as “RA”) is a common human autoimmune disease. The current understanding of the pathophysiology of RA is that autoimmune T cells and B cells initiate an inflammatory response in the joints, which drives hyperproliferation of the synoviocytes. As a consequence of the hyperproliferation of synovial cells, metalloproteinases (hereinafter referred to as “MMPs”) are over-produced, which further leads to the erosive destruction of the cartilage and bone that is characteristic of RA (25). Thus, the control of hyperproliferation of inflammatory synovial cells is a key step in the treatment of RA. The molecular mechanisms leading to the hyperproliferation of synovial cells are still unknown. Although the hyperproliferative synovial cells are non-malignant and non-transformed, many studies have suggested that they share some common features with transformed cells (46). These cells, the so-called, “transformed-appearing synoviocytes”, are characterized by a dense rough endoplasmic reticulum, numerous irregular nuclei, and changes in the normally spindle-shaped cell skeleton. It has been proposed that the incorporation of the oncogenes and virus-derived genes might be the primary triggers for the transformed appearance of RA synovial cells (46).
At least two aspects of RA suggest that dysregulated apoptosis may contribute to the disease process and that therapeutic elicitation of apoptosis may be an effective treatment: the failure of the deletion of the activated T cells suggests that there is defective activation-induced cell death of these T cells, which is a process that involves Fas-mediated apoptosis and TRAIL-mediated apoptosis, and the hyperproliferative nature of the RA synovial cells is a contributing factor in the later stages of RA pathophysiology. Indeed, it has been shown that the administration of anti-Fas antibody into the inflammatory joint inhibits the development of chronic arthritis in tax transgenic mice, which are an animal model for human RA (26). Moreover, localized transduction with the fas ligand gene by an adenoviral vector is effective in prevention of collagen-induced arthritis (27). Inhibition of the proliferation of inflammatory synovial cells by enhancement of Fas-mediated apoptosis is observed in both cases. Although Fas ligand is a strong apoptosis inducer in RA synovial cells, the application of Fas ligand-mediated apoptosis as a therapy for humans has been limited by lethal liver toxicity. Thus, TRAIL receptor induced apoptosis represents a safer and more effective therapeutic for the treatment of RA than Fas-ligand induced apoptosis.
TRAIL receptor induced apoptosis also represents a safer and more effective therapeutic for the treatment of cancer than Fas-ligand induced apoptosis. TRAIL mediated apoptosis is known to specifically induce apoptosis of transformed tumor cells without affecting normal cells. It has been shown that the systemic administration of the trimerized soluble TRAIL did not cause toxicity in experimental animals yet was able to induce regression of implanted tumors (13,28). Its potential as an adjunctive therapy for traditional treatments was underscored by the recent finding that the expression of DR5 and susceptibility to TRAIL-induced apoptosis of breast cancer cells is enhanced by the radiation, suggesting that combined with radiation, the efficiency of TRAIL would be increased in cancer therapy (29).
In addition, the gene encoding the TRAIL receptor DR5 has been mapped to chromosome 8p21–22, a loci with a high frequency of mutation in some cancer cells (30). It has been reported that at least two kinds of tumor cells, small lung cancer (31) and head and neck cancer (32) exhibit mutations in the death domain of the DR5 gene. Thus, there exists a need for an anti-DR5 antibody in cancer research to determine the effect of receptor epitope variation on the development and progression of cancers. Further, the functionality of TRAIL receptor mutations would prove a useful clinical diagnostic tool when used in conjunction with other biomarkers in the early detection of cancers and as a predictor of the tumor aggressiveness.